1. Technical Field
The present invention relates to an amplifier circuit that can be applied to a sensor circuit on a sensor substrate used for inspection of a substrate for display such as a glass substrate of a liquid crystal display panel, for example.
2. Description of the Related Art
Background Art
The substrate for display is, for example, a multiple-piece glass substrate provided with a plurality of substrate regions for display, each of which is divided into a liquid crystal display panel, on one face. As shown in FIG. 39, each substrate region 10 for display has a large number of pixel regions (that is, cell regions) provided with a pixel electrode 12 in a rectangular shape and a switching element 14 connected to the pixel electrode 12 in a matrix state.
Each pixel electrode 12 is a thin-film state electrode parallel with the substrate 10 for display and has a rectangular plane shape having substantially the same size as that of a corresponding pixel region, for example. Each switching element 14 is a field effect thin film transistor (TFT) having a source electrode, a drain electrode and a gate electrode, for example, and the drain electrode (or the source electrode) is connected to the corresponding pixel electrode 12. The gate electrode of the switching element 14 aligned in the X direction is connected to a common gate wiring 16, while the source electrode (or the drain electrode) of the switching element 14 aligned in the Y direction is connected to a common wiring 18.
By turning on the switching elements 14 of the applicable row by voltage control of the gate wiring 16 and applying a high-frequency signal for test to the wiring 18 so as to charge and discharge the pixel electrode 12 of the applicable row and the like, disconnection of the switching element 14, the gate wiring 16, and the wiring 18 can be detected. The pixel electrodes 12 are arranged adjacently in 7168 pieces in (one row of) the X direction, for example, and inspection for disconnection and the like is conducted for each row. One row in the X direction has a length of 25 cm and some, for example.
Japanese Patent Laid-Open No. 2007-248202 describes a method of inspection by opposing a sensor substrate to a row of the pixel electrode 12 to be inspected in a non-contact manner.
On the sensor substrate, sensor electrodes to be opposed to the pixel electrodes 12 on a one-on-one manner are aligned with a pitch similar to the array of the pixel electrodes 12 in the X direction. At an approximate distance where the pixel electrode 12 and the corresponding sensor electrode are electromagnetically coupled, the sensor substrate is brought close to the row of the pixel electrode 12 to be inspected, a signal radiated from the pixel electrode 12 (the above-mentioned high-frequency signal for test) is picked up by the sensor electrode and amplified through an amplifier circuit for sensing (See FIG. 8 in Japanese Patent Laid-Open No. 2007-248202) and then, presence of the signal and the like is checked by a tester portion for inspection.
By intermittently and relatively moving the substrate for display and the sensor substrate, each row of the pixel electrodes 12 is sequentially inspected.
For example, since the pixel electrodes 12 are arranged adjacently in 7168 pieces for the length of 25 cm and some as mentioned above, it is necessary to install 7168 pieces of the amplifier circuits for sensing formed on the sensor substrate for the length of only 25 cm and some, for example. Therefore, it is practical that the amplifier circuit for sensing is constituted by SOG (polysilicon), and it is required that the amplifier circuit has high input impedance since it has a micro capacitor coupling input, amplification characteristics (gain, output bias and the like) are not varied even in the case of a power supply voltage drop due to characteristic variation of the element or a long power supply line resistance of 25 cm and some since a large number can be arranged adjacently, and an actual circuit area when being made into an IC is small and the like, and use of a source grounding amplifier circuit as shown in FIG. 40 in each amplifier circuit has been examined, for example.
In FIG. 40, a grounded-source amplifier circuit 20 is configured such that a source resistance for negative feedback Rs is connected between a source of an amplifier MOS transistor M1 connecting the gate to an input terminal Vi of the grounded-source amplifier circuit 20 and a negative power supply Vee, a load resistance RL is connected between a drain of the MOS transistor M1 and a positive power supply Vdd, and a drain connection end of the amplifier MOS transistor M1 of the load resistance RL is constituted as an output terminal Vo of the grounded-source amplifier circuit 20. An input terminal Vi of the grounded-source amplifier circuit 20 is connected to an output Vso of a signal source 22. FIG. 40 shows a signal source 22 in an equivalent circuit, regarding a signal picked up by the above-mentioned sensor electrode as a signal from the signal source 22. The signal source 22 is configured such that an input direct-current bias power supply Vidc and an input alternating current signal source Vs are connected in series, and one end of the series circuit is connected to a ground, while the other end is made as the signal source output Vso. Any of the positive power supply Vdd, the negative power supply Vee, and the input direct-current bias power supply Vidc of the signal source 22 may be connected to OV (that is, the ground).
In the grounded-source amplifier circuit 20, since the gate of the MOS transistor M1 is the input terminal Vi of the grounded-source amplifier circuit 20, an electric current does not flow into the input terminal Vi.
On the other hand, a direct current with a value obtained by dividing a difference of direct-current potential between the input terminal Vi and the negative power supply Vee by the sum of a direct-current source resistance of the MOS transistor M1 and the source resistance for negative feedback Rs flows into the source and the drain of the MOS transistor M1, and an alternating current (signal current) with a value obtained by dividing a voltage of the input alternating current signal source Vs by the sum of an alternating current source impedance of the MOS transistor M1 and the source resistance for negative feedback Rs flows into the source and the drain of the MOS transistor M1.
Then, the product of the drain alternating current (output signal current) and the load resistance RL becomes an output voltage.
As mentioned above, a voltage gain A when the input impedance of a subsequent-stage circuit connected to the output Vo of the grounded-source amplifier circuit 20 is infinite is expressed by an expression (1), supposing that the alternating source impedance of the MOS transistor M1 is RM1s:A=RL/(RM1s+Rs)  (1)
In the case of RM1s≈Rs, a variation of the source impedance RM1s of the amplifier MOS transistor M1 directly leads to a variation in the gain.
Here, if RM1s is sufficiently smaller than Rs, an expression (2) is true, but RM1s can not be ignored in general and is handled by the expression (1).A≈RL/Rs  (2)